This invention relates to keyboards and, more particularly, to interchangeable key pad assemblies for providing at least a portion of a user keyboard. Specifically, one embodiment of the invention provides interchangeable key pad assemblies that are coded so that when they are alternately installed on a keyboard, one of a variety of programmable sets of keyboard functions is enabled for selection by user actuation of the keys.
One exemplary use of the invention is in association with molded rubber keyboards. Molded rubber keyboards have been used in combination with underlying printed circuit boards to provide a keyboard structure much less expensive than those where each key requires a separate push button member separately mounted in a frame. The term "rubber," as used in this specification and in the claims, refers not only to synthetic and natural rubbers, but also to other elastomeric materials.
Molded rubber keyboards usually comprise a rubber sheet integral with upwardly projecting rubber keys. Each rubber key is located above a recess in the sheet and its lower end includes such means as conductive rubber members for closing the switch contacts on the printed circuit board located directly below it. The recesses form a diaphragm which acts to space the conductive rubber members above the printed circuit board, except when a particular key is pressed.
Church, U.S. Pat. No. 4,764,770, for example, discloses a keyboard assembly which has a printed circuit board providing a series of switch contacts and a molded rubber keyboard providing a rubber base sheet overlying the printed circuit board and having a series of rubber keys integral with the base sheet. Each key has contacts on its bottom surface for closing each of the switch contacts when its key is depressed. Stabilizing membranes prevent the keys from wobbling or binding. Although undesirable wobbling or binding is substantially eliminated, the keyboard is either a dedicated keyboard or one whose key functions are determined by the system to which the keyboard is mounted.
Previously, overlays have been used to provide the user with identification of key functions for programmable or customizable key sets. Overlays are commonly used on computer keyboards and calculators. The typical overlay construction is a thin plastic sheet with screen-printed information. This material is not very durable and can be easily damaged. Furthermore, these overlays typically are not securely attached to the computer keyboards and calculators. Consequently, they are easily dislodged and slide from the keyboard, frustrating the user. Also, the functions of the keys are dedicated or determined by the computer or calculator since the overlay does not operatively interact with the system.
It is desirable to overcome the disadvantages of overlays by providing a more durable and secure means for identifying key functions. Furthermore, it is also desirable to interact with the system to identify which of a variety of sets of key functions is enabled for selection by the user.